Transverse tripole stimulation (“TTS”) may involve, for example, at least three electrodes arranged substantially along a line approximately perpendicular to the axis of the spinal cord. The electrical field can be steered from side to side by varying the current or voltage between the center electrode and the outer electrodes. Voltages or currents can be in phase (overlapping in time) or out of phase between the right and left side. Using the outer electrodes as anodes may prevent nerve root stimulation.
See, e.g., U.S. Pat. Nos. 5,501,703; 5,643,330 and 5,895,416.